为什么Mvvm方法不适合我的计算器

Question

我正在尝试使用silverlight 5和我的xaml代码是这样的MVVM工作:

 <Grid x:Name="LayoutRoot" Background="White" Height="100" Width="350">
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
       <TextBox Grid.Column="0" Text="{Binding FirstValue, Mode=TwoWay}" Height="25" TextAlignment="Right"/>
        <TextBlock Grid.Column="1" Text="+" Height="25" TextAlignment="Center"/>
        <TextBox Grid.Column="2" Text="{Binding SecondValue, Mode=TwoWay}" Height="25" TextAlignment="Right"/>
        <TextBlock Grid.Column="3" Text="=" Height="25" TextAlignment="Center"/>
        <TextBlock Grid.Column="4" Text="{Binding Result, Mode=OneWay}" Height="25" TextAlignment="Left"/>
        <Button Grid.Row="1" Grid.ColumnSpan="5" Margin="0,5,0,0" Content="Calculate" Command="{Binding CalculateCommand}"  />
    </Grid>

和

public MainPage()
        {
            InitializeComponent();
            this.DataContext = new CalculatorViewModel();
        }

我的CalculatorViewModel.cs类是:

public class CalculatorViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        public CalculatorModel cc;

        private string _firstValue;
        public string FirstValue
        {
            get { return _firstValue; }
            set
            {
                _firstValue = value;
                OnPropertyChanged("FirstValue");
            }
        }
        private string _secondValue;
        public string SecondValue
        {
            get { return _secondValue; }
            set
            {
                _secondValue = value;
                OnPropertyChanged("SecondValue");
            }
        }
        private string _result;
        public string Result
        {
            get { return _result; }
            private set
            {
                _result = value;
                OnPropertyChanged("Result");
            }
        }
        private ICommand _calculateCommand;
        public ICommand CalculateCommand
        {
            get { return _calculateCommand; }
        }
        public CalculatorViewModel()
        {
            cc = new CalculatorModel();
            cc.FirstValue = _firstValue;
            cc.SecondValue = _secondValue;
            cc.Result = _result;
            _calculateCommand = new RelayCommand(cc.Calculate) { IsEnabled = true };
            //This Calculate() is defined in Model class (CalculatorModel.cs).
         }    
        protected void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this,new PropertyChangedEventArgs(propertyName));
            }
        }
    }

我想在模型类(CalculatorModel.cs)中定义这个计算委托。根据我的理解，这是什么模型类的意思。(如果我错了请纠正我)。

我这样做了:

 public class CalculatorModel
    {
        #region PROPERTIES
        public string FirstValue;
        public string SecondValue;
        public string Result;
        #endregion
        //CalculatorViewModel cv = new CalculatorViewModel();
        public CalculatorModel()
        {
            this.Result = "";
            this.FirstValue = "";
            this.SecondValue = "";              
        }
        public void Calculate()
        {
            MessageBox.Show("check : " +FirstValue);
            Result = (Convert.ToInt32(FirstValue) + Convert.ToInt32(SecondValue)).ToString();
        }
    }

问题是我不知道为什么在调试结果，FirstValue和SecondValue的值为空委托calculate()被称为按钮单击，但没有在结果中更新的输出。

有人能纠正我在模型类中定义calculate()的方法有什么问题吗?

Answer 1

您似乎没有更新模型中的值。数据绑定只在View和ViewModel之间工作。

    private string _firstValue;
    public string FirstValue
    {
        get { return _firstValue; }
        set
        {
            _firstValue = value;
            cc.FirstValue = _firstValue;     // add this
            OnPropertyChanged("FirstValue");
        }
    }

，对SecondValue也这样做。修改命令，将结果赋值给result。

这告诉您，您的设置不是最有效或最优雅的。考虑

1. 使模型INotifyPropertyChanged并绑定到模型。FirstValue
2. 或者，在这里放弃模型的概念。

当你使模型实现INPC时，你可以(应该)完全从VM中删除FirstValue, SecondValue和Result。您可以绑定到Model: Text="{Binding Path=cc.FirstValue, Mode=TwoWay}"，然后使Model成为一个属性:

   public CalculatorModel cc { get; set; }   // must be a property

否则，"重复"属性的更好模式是:

    //private string _firstValue;
    public string FirstValue
    {
        get { return cc.FirstValue; }
        set
        {
            //_firstValue = value;
            cc.FirstValue = value;           // add this
            OnPropertyChanged("FirstValue");
        }
    }